CoAsy knockdown in TNBC cell lines resulted in no overt effect on cell proliferation in vitro.

Triple-negative breast cancer (TNBC) remains the most challenging breast cancer subtype to treat. CoA synthase (CoAsy) is a bifunctional enzyme, encoded by the COASY gene, which catalyzes the last two steps of CoA biosynthesis. COASY has been reported as a hit in several large RNAi library screens for cancer. Therefore, we sought to investigate the dependency of TNBC cell line proliferation on CoAsy expression. Initially, knockdown of CoAsy expression was achieved by RNAi and reduced proliferation was observed in two TNBC cell lines, HCC1806 and MDA-MB-231. To further investigate the role of CoAsy, we established stable inducible shRNA cell lines from the same TNBC cell lines as well as the normal-like breast cell line MCF10A. Three separate cell lines, each expressing one of three different shRNA constructs targeting COASY, and a non-targeted shRNA control cell line were generated from each parent cell line. The induction of COASY shRNA for 4 days resulted in >99% knockdown of CoAsy for all three COASY shRNA constructs. However, this robust knockdown of CoAsy protein expression had no detectable impact on cell growth with 4-day induction times. Even 8-day induction times resulted in no apparent impact on cell growth. There was also no effect of CoAsy knockdown on the rate of cell migration. Measurement of CoA levels in cell lysates indicated that CoAsy knockdown reduced CoA to approximately half the normal level. Thus, CoAsy knockdown showed no detectable effect on the in vitro proliferation and migration of these cell lines possibly due to the cell's ability to maintain adequate levels of CoA through some unknown mechanism.

Discovery and characterization of an iminocoumarin scaffold as an inhibitor of MEKK2 (MAP3K2).

The kinase MEKK2 (MAP3K2) activates the MEK5/ERK5 cell signaling pathway and may play an important role in tumor growth and metastasis. Thus, MEKK2 may represent a novel kinase target for cancer. In order to identify inhibitors of MEKK2, we screened a library of compounds using a high throughput MEKK2 intrinsic ATPase enzyme assay. We identified two hits with validated structures and confirmed activity in the primary assay (IC50 values = 322 nM and 7.7 µM) and two orthogonal MEKK2 biochemical assays. Compound 1, the more potent hit, was the subject of further investigation. Limited structure-activity relationship (SAR) studies were performed on this iminocoumarin hit which resulted in ≥20-fold more potent analogs (e.g. 8 and 16 nM IC50). Two analogs had improved selectivity in a 50-member kinase profiling panel compared to the hit. These studies suggested that substitutions around the phenoxy ring of this scaffold can impart improved potency and selectivity for MEKK2. Analog Compound 1s (16 nM IC50) was further verified by external testing to inhibit MEKK2 and MEKK3 with similar potencies. Compound 1s displayed activity in cell-based assays in which it inhibited ERK5 pathway activation in cells and inhibited cell migration in a scratch assay. Thus, we have identified a scaffold that has promising potential to be developed into a highly selective and potent inhibitor of MEKK2. Information from these SAR studies provides specific guidance for the future design of MEKK2 inhibitor probes.

Identification of ponatinib and other known kinase inhibitors with potent MEKK2 inhibitory activity.

The kinase MEKK2 (MAP3K2) may play an important role in tumor growth and metastasis for several cancer types. Thus, targeting MEKK2 may represent a novel strategy for developing more effective therapies for cancer. In order to identify small molecules with MEKK2 inhibitory activity, we screened a collection of known kinase inhibitors using a high throughput MEKK2 intrinsic ATPase enzyme assay and confirmed activity of the most potent hits with this primary assay. We also confirmed activities of these known kinase inhibitors with an MEKK2 transphosphorylation slot blot assay using MKK6 as a substrate. We observed a good correlation in potencies between the two orthogonal MEKK2 kinase activity assay formats for this set of inhibitors. We report that ponatinib, AT9283, AZD7762, JNJ-7706621, PP121 and hesperadin had potent MEKK2 enzyme inhibitory activities ranging from 4.7 to 60 nM IC50. Ponatinib is an FDA-approved drug that potently inhibited MEKK2 enzyme activity with IC50 values of 10-16 nM. AT9283 is currently in clinical trials and produced MEKK2 IC50 values of 4.7-18 nM. This set of known kinase inhibitors represents some of the most potent in vitro MEKK2 inhibitors reported to date and may be useful as research tools. Although these compounds are not selective for MEKK2, the structures of these compounds give insight into pharmacophores that potently inhibit MEKK2 and could be used as initial leads to design highly selective inhibitors of MEKK2.

A high throughput proliferation and cytotoxicity assay for co-cultured isogenic cell lines.

PTEN is a well-known tumor suppressor that is inactivated or suppressed at a high frequency in cancer. We sought an assay to screen compounds for ones that differentially inhibited proliferation or induced cytotoxicity in PTEN mutated cancer cells. We employed the isogenic pair of cell lines MCF10-A breast cell line (wild type, WT) and the same cell line with PTEN knocked out (KO) by CRISPR. We sought an assay where these PTEN WT and KO isogenic cell lines were co-cultured in the same well for compound testing. The KO cell line, but not the WT, was tagged with the red fluorescent protein mKate2. We employed a real time microscopic imaging instrument to identify cell populations in co-culture based on red fluorescence to obtain a cell count for each cell line. To acquire cytotoxicity data for each population, the dye CellTox Green was added to the media. To assess the assay, we determined the concentration response of paclitaxel. In order to assess the potential for screening, we performed mock screening in 384-well plate format. Thus, we developed a high throughput co-culture cell cytotoxicity and proliferation assay method that could be employed for any pair of cell lines to identify selective compounds.

Estradiol enhances cell-associated paraoxonase 1 (PON1) activity in vitro without altering PON1 expression.

PON1 is a high density lipoprotein-associated enzyme that plays an important role in organophosphate detoxification and prevention of atherosclerosis. In vivo animal and human studies have indicated that estradiol (E2) supplementation enhances serum PON1 activity. In this study, we sought to determine if E2 directly up-regulates cell-associated PON1 activity in vitro and to characterize the mechanism of regulation. In vitro E2 treatment of both the human hepatoma cell line Huh7 and normal rat hepatocytes resulted in a 2- to 3-fold increase in cell-associated PON1 catalytic activity. E2 potently induced PON1 activity with average EC(50) values of 15nM for normal hepatocytes and 68nM for Huh7. The enhancement of PON1 activity by E2 was blocked by the estrogen receptor (ER) antagonist ICI 182,780 indicating that E2 was acting through the ER. The up-regulation of PON1 activity by E2 did not involve enhancement of PON1 mRNA or protein levels and did not promote secretion of PON1. Thus, E2 can enhance cell-associated PON1 activity in vitro without altering PON1 gene expression or protein level. Our data suggest that E2 may regulate the specific activity and/or stability of cell surface PON1.

A homogeneous cell-based assay for measurement of endogenous paraoxonase 1 activity.

Paraoxonase 1 (PON1) is a high-density lipoprotein-associated enzyme that plays an important role in organophosphate detoxification and prevention of atherosclerosis. Thus, there is significant interest in identifying nutritional and pharmacological enhancers of PON1 activity. To identify such compounds, we developed a rapid homogeneous assay to detect endogenous cell-associated PON1 activity. PON1 activity was measured by the simple addition of fluorigenic PON1 substrate DEPFMU to live Huh7 cells in medium and monitoring change in fluorescence. A specific PON1 inhibitor, 2-hydroxyquinoline, was used to confirm that the observed activity was due to PON1. The assay was optimized and characterized with regard to time course, substrate and sodium chloride concentration, number of cells, and tolerance to dimethyl sulfoxide and serum. Aspirin, quercetin, and simvastatin are compounds reported to increase PON1 expression. Consistent with the literature and Western blot data, these compounds enhanced PON1 activity in this assay with comparable efficacies and potencies. A known toxic compound did not increase assay signal. This assay method also detected PON1 activity in normal hepatocytes. Thus, a novel homogeneous assay for detection of endogenous PON1 expression has been developed and is amenable to high-throughput screening for the identification of small molecules that enhance PON1 expression.

Identification of a Triple Drug Combination That Is Synergistically Cytotoxic for Triple-Negative Breast Cancer Cells Using a Novel Combination Discovery Approach.

Triple-negative breast cancer (TNBC) is a very aggressive form of breast cancer with few molecularly targeted therapies. We used a novel unbiased approach to identify higher-order synergistic or enhancer combinations of marketed kinase inhibitor drugs that inhibit cell viability of TNBC cell lines. We mixed all 33 kinase-targeted drugs on the market at the time of this study, which allowed for all possible combinations to exist in the initial mixture. A kinase inhibitor group dropout approach was used to identify active groups and then single active drugs. After only three rounds of deconvolution, we identified five single drugs to test further. After further testing, we focused on one novel subset consisting of three kinase inhibitor drugs: dasatinib, afatinib, and trametinib (DAT) that target src family kinases, HER2/EGFR, and MEK, respectively. The DAT combination potently inhibited the proliferation of three TNBC cell lines and modestly inhibited a fourth. However, it was not significantly more potent or synergistic than other two drug combinations of these drugs. The cytotoxic activities of all possible combinations of these three drugs were also analyzed. Compared with all two-way combinations, the three-way DAT combination generated the most cytotoxicity and the highest synergies for two of the four cell lines tested, with possibly mild synergy in a third cell line. These data indicated that the DAT combination should be evaluated for efficacy in an in vivo model of TNBC and may provide a novel combination of existing drugs for the treatment of a subset of TNBC cases.

MEKK2 mediates aberrant ERK activation in neurofibromatosis type I.

Neurofibromatosis type I (NF1) is characterized by prominent skeletal manifestations caused by NF1 loss. While inhibitors of the ERK activating kinases MEK1/2 are promising as a means to treat NF1, the broad blockade of the ERK pathway produced by this strategy is potentially associated with therapy limiting toxicities. Here, we have sought targets offering a more narrow inhibition of ERK activation downstream of NF1 loss in the skeleton, finding that MEKK2 is a novel component of a noncanonical ERK pathway in osteoblasts that mediates aberrant ERK activation after NF1 loss. Accordingly, despite mice with conditional deletion of Nf1 in mature osteoblasts (Nf1fl/fl;Dmp1-Cre) and Mekk2-/- each displaying skeletal defects, Nf1fl/fl;Mekk2-/-;Dmp1-Cre mice show an amelioration of NF1-associated phenotypes. We also provide proof-of-principle that FDA-approved inhibitors with activity against MEKK2 can ameliorate NF1 skeletal pathology. Thus, MEKK2 functions as a MAP3K in the ERK pathway in osteoblasts, offering a potential new therapeutic strategy for the treatment of NF1.

Enzyme assay design for high-throughput screening.

Enzymes continue to be a major drug target class for the pharmaceutical industry with high-throughput screening the approach of choice for identifying initial active chemical compounds. The development of fluorescent- or absorbance-based readouts typically remains the formats of choice for enzyme screens and a wealth of experience from both industry and academia has led to a comprehensive set of standardized assay development and validation guidelines for enzyme assays. In this chapter, we generalize approaches to developing, validating, and troubleshooting assays that should be applicable in both industrial and academic settings. Real-life examples of various enzyme classes including kinases, proteases, transferases, and phosphatases are used to illustrate assay development approaches and solutions. Practical examples are given for how to deal with low-purity enzyme targets, compound interference, and identification of activators. Assay acceptance criteria and a number of assay notes on pitfalls to avoid should provide pointers on how to develop a suitable enzymatic assay applicable for HTS.

Homogeneous time-resolved fluorescence resonance energy transfer assay for measurement of Phox/Bem1p (PB1) domain heterodimerization.

Twenty human proteins encode Phox/Bem1p (PB1) domains, which are involved in forming protein heterodimers. MEKK2, MEKK3, and MEK5 are 3 serine-threonine protein kinases that have PB1 domains. MEKK2, MEKK3, and MEK5 are the MAP3Ks and the MAP2K in the ERK5 mitogen-activated protein kinase (MAPK) signaling module. ERK5 is a critical MAPK for both development of the vasculature and vascular homeostasis in the adult, but no other MAPK has been shown to be critical in vascular maintenance in the adult animal. MEKK2 and MEKK3 are the only MAP3Ks shown to physically interact with and activate the MEK5-ERK5 signaling module. Interaction of MEKK2 or MEKK3 with MEK5 is mediated by heterodimerization of the MEKK2 (or MEKK3) PB1 and MEK5 PB1 domains. The authors have developed a homogeneous, time-resolved fluorescence resonance energy transfer (TR-FRET) assay to monitor PB1-PB1 domain heterodimerization. The assay uses a europium-chelate conjugated GST-MEK5 PB1 domain chimera, biotinylated MEKK2 PB1 domain, and streptavidin-Cy5. Interaction of the MEKK2 and MEK5 PB1 domains gives a robust FRET signal (Z' factor = 0.93), which is completely abrogated by mutation of 2 acidic residues (64D65E-->AA) within the MEK5 PB1 domain that causes loss of stable PB1-PB1 domain interaction. This assay can be used to study the specificity of PB1-PB1 domain interactions and to screen for molecules that can regulate MEKK2/MEKK3-MEK5 interactions. Disruption of PB1 domain interactions represents a novel approach for selectively regulating the ERK5 signaling pathway independent of kinase active site-directed adenosine triphosphate competitive inhibitors.

Cartilage is held together by elastic glycan strings. Physiological and pathological implications.

Animal shapes are maintained by connective tissue extracellular matrices (ECMs). ECM shapes depend on keeping collagen fibrils in the right places, held by regular frequent proteoglycan (PG) bridges attached at specific sites. The PGs carry anionic glycosaminoglycan (AGAG) 'strings' that span and determine interfibrillar distances, thus holding us together. I called these repeating structures 'shape modules'. The strings are aggregated antiparallel chains of dermochondan, keratan and chondroitan sulphates (DS, KS and CS); stabilised by hydrophobic and H-bonds. Shape modules are elastic. AGAG/AGAG interactions break under stress and reform when the stress is removed and/or they contain an elastic sugar, L-iduronate (in DS). Cartilage ECMs are also based on shape modules. Depots therein of aggrecan, the large PG which carries many chains of CS and KS, imbibe water, thereby exerting swelling pressure. External pressure forces this water into the elastic shape modules, from whence it is returned post compression. Cartilage anisotropic responses (along and at right angles to shape module axes) to compressive and tensile stresses are now interpretable. Degradation of shape modules in osteoarthrosis reduces these responses. Inability to hold collagen fibrils together results in imbibition of excess water, fissuring and erosion, characteristic of this condition.

Toward the effective surveillance of hypospadias.

Concern about apparent increases in the prevalence of hypospadias--a congenital male reproductive-tract abnormality--in the 1960s to 1980s and the possible connection to increasing exposures to endocrine-disrupting chemicals have underlined the importance of effective surveillance of hypospadias prevalence in the population. We report here the prevalence of hypospadias from 1980 to 1999 in 20 regions of Europe with EUROCAT (European Surveillance of Congenital Anomalies) population-based congenital anomaly registers, 14 of which implemented a guideline to exclude glanular hypospadias. We also report data from the England and Wales National Congenital Anomaly System (NCAS). Our results do not suggest a continuation of rising trends of hypospadias prevalence in Europe. However, a survey of the registers and a special validation study conducted for the years 1994-1996 in nine EUROCAT registers as well as NCAS identified a clear need for a change in the guidelines for registration of hypospadias. We recommend that all hypospadias be included in surveillance, but that information from surgeons be obtained to verify location of the meatus, and whether surgery was performed, in order to interpret trends. Investing resources in repeated special surveys may be more cost-effective than continuous population surveillance. We conclude that it is doubtful whether we have had the systems in place worldwide for the effective surveillance of hypospadias in relation to exposure to potential endocrine-disrupting chemicals.

Bosutinib reduces the efficacy of Dasatinib in triple-negative breast cancer cell lines.

BACKGROUND: Triple-negative breast cancer (TNBC) is an aggressive sub-type of breast cancer. Dasatinib and bosutinib are FDA-approved Src/Abl kinase inhibitor drugs. Dasatinib potently inhibits the proliferation of many TNBC cell lines. MATERIALS AND METHODS: The cell viability/proliferation for a panel of 4 TNBC cell lines was measured by detection of cellular ATP levels and cell numbers were directly determined by automated cell counting. RESULTS: Bosutinib (≤1 µM) had little to no inhibitory activity on cell viability/proliferation, while dasatinib-alone generated potent IC50 values of <100 nM. Combination treatment of cells with both dasatinib and bosutinib resulted in reduced efficacy of dasatinib in all four cell lines, with two of them displaying a dramatic loss of efficacy. Direct cell counting confirmed that bosutinib enhanced cell proliferation in the presence of dasatinib. CONCLUSION: Bosutinib potently reduced the in vitro anti-proliferative efficacy of dasatinib in TNBC cell lines. We, hereby, report on a novel drug-induced loss in dasatinib sensitivity.

Old drug new use--amoxapine and its metabolites as potent bacterial ß-glucuronidase inhibitors for alleviating cancer drug toxicity.

PURPOSE: Irinotecan (CPT-11) induced diarrhea occurs frequently in patients with cancer and limits its usage. Bacteria ß-glucuronidase (GUS) enzymes in intestines convert the nontoxic metabolite of CPT-11, SN-38G, to toxic SN-38, and finally lead to damage of intestinal epithelial cells and diarrhea. We previously reported amoxapine as a potent GUS inhibitor in vitro. To further understand the molecular mechanism of amoxapine and its potential for treatment of CPT-11-induced diarrhea, we studied the binding modes of amoxapine and its metabolites by docking and molecular dynamics simulation, and tested the in vivo efficacy on mice in combination with CPT-11. EXPERIMENTAL DESIGN: The binding of amoxapine, its metabolites, 7-hydroxyamoxapine and 8-hydroxyamoxapine, and a control drug loxapine with GUS was explored by computational protocols. The in vitro potencies of metabolites were measured by Escherichia coli GUS enzyme and cell-based assay. Low-dosage daily oral administration was designed to use along with CPT-11 to treat tumor-bearing mice. RESULTS: Computational modeling results indicated that amoxapine and its metabolites bound in the active site of GUS and satisfied critical pharmacophore features: aromatic features near bacterial loop residue F365' and hydrogen bond toward E413. Amoxapine and its metabolites were demonstrated as potent in vitro. Administration of low dosages of amoxapine with CPT-11 in mice achieved significant suppression of diarrhea and reduced tumor growth. CONCLUSIONS: Amoxapine has great clinical potential to be rapidly translated to human subjects for irinotecan-induced diarrhea.

Development and validation of a high-throughput intrinsic ATPase activity assay for the discovery of MEKK2 inhibitors.

The kinase MEKK2 (MAP3K2) has recently been implicated in tumor growth and metastasis. Thus, selective inhibition of MEKK2 may be a novel strategy for cancer therapy. To identify inhibitors of MEKK2 kinase activity, we have developed a novel activity assay for MEKK2 based on the discovery that recombinant purified MEKK2 has intrinsic ATPase activity. This MEKK2 ATPase assay was validated for enzyme identity and enzymatic purity by multiple methods including mass spectrometry analysis, testing different sources of MEKK2 and comparing ATPase assay IC50 data for multiple inhibitors to literature values and to IC50 data generated using MEKK2 binding and transphosphorylation assays. Taken together, these data indicated that genuine MEKK2 activity was being measured in this assay and no other ATPases contributed to the signal. A miniaturized version of the assay was validated for high-throughput screening, and compound libraries were screened. The screening hits generated comparable potencies in the MEKK2 intrinsic ATPase, binding, and transphosphorylation assays. We identified a novel MEKK2 inhibitor and confirmed that crizotinib and bosutinib are potent in vitro inhibitors of MEKK2 activity with IC50 values of <100 nM. Thus, this assay has utility for the discovery of small-molecule inhibitors of MEKK2 activity.

Potential repurposing of known drugs as potent bacterial ß-glucuronidase inhibitors.

The active metabolite of the chemotherapeutic irinotecan, SN-38, is detoxified through glucuronidation and then excreted into the gastrointestinal tract. Intestinal bacteria convert the glucuronidated metabolite back to the toxic SN-38 using ß-glucuronidase (GUS), resulting in debilitating diarrhea. Inhibiting GUS activity may relieve this side effect of irinotecan. In this study, we sought to determine whether any known drugs have GUS inhibitory activity. We screened a library of Food and Drug Administration-approved drugs with a cell-free biochemical enzyme assay using purified bacterial GUS. After triage, five drugs were confirmed to inhibit purified bacterial GUS. Three of these were the monoamine oxidase inhibitors nialamide, isocarboxazid, and phenelzine with average IC(50) values for inhibiting GUS of 71, 128, and 2300 nM, respectively. The tricyclic antidepressant amoxapine (IC(50) = 388 nM) and the antimalarial mefloquine (IC(50) = 1.2 µM) also had activity. Nialamide, isocarboxazid, and amoxapine had no significant activity against purified mammalian GUS but showed potent activity for inhibiting endogenous GUS activity in a cell-based assay using living intact Escherichia coli with average IC(50) values of 17, 336, and 119 nM, respectively. Thus, nialamide, isocarboxazid, and amoxapine have potential to be repurposed as therapeutics to reduce diarrhea associated with irinotecan chemotherapy and warrant further investigation for this use.

A High Throughput Assay for Discovery of Bacterial ß-Glucuronidase Inhibitors.

CPT-11 is a widely-used anti-cancer drug that is converted in vivo to its active metabolite, SN-38. In the liver, enzymes detoxify SN-38 by coupling it to a glucuronidate moiety and this inactive compound (SN-38G) is excreted into the gastrointestinal tract. In the intestine, commensal bacteria convert the SN-38G back to the active and toxic SN-38 using bacterial ß-glucuronidase enzyme (GUS). This intestinal SN-38 causes debilitating diarrhea that prevents dose-intensification and efficacy in a significant fraction of patients undergoing CPT-11 treatment for cancer. This CPT-11 metabolic pathway suggests that small molecule inhibitors of GUS may have utility as novel therapeutics for prevention of dose-limiting diarrhea resulting from CPT-11 therapy. To identify chemical inhibitors of GUS activity, we employed and validated a high throughput, fluorescence-based biochemical assay and used this assay to screen a compound library. Novel inhibitors of GUS were identified with IC(50) values ranging from 50 nM to 4.8 µM. These compounds may be useful as chemical probes for use in proof-of-concept experiments designed to determine the efficacy of GUS inhibitors in altering the intestinal metabolism of drugs. Our results demonstrate that this high throughput assay can be used to identify small molecule inhibitors of GUS.

Alleviating cancer drug toxicity by inhibiting a bacterial enzyme.

The dose-limiting side effect of the common colon cancer chemotherapeutic CPT-11 is severe diarrhea caused by symbiotic bacterial ß-glucuronidases that reactivate the drug in the gut. We sought to target these enzymes without killing the commensal bacteria essential for human health. Potent bacterial ß-glucuronidase inhibitors were identified by high-throughput screening and shown to have no effect on the orthologous mammalian enzyme. Crystal structures established that selectivity was based on a loop unique to bacterial ß-glucuronidases. Inhibitors were highly effective against the enzyme target in living aerobic and anaerobic bacteria, but did not kill the bacteria or harm mammalian cells. Finally, oral administration of an inhibitor protected mice from CPT-11-induced toxicity. Thus, drugs may be designed to inhibit undesirable enzyme activities in essential microbial symbiotes to enhance chemotherapeutic efficacy.

A high throughput assay to identify small molecule modulators of prostatic acid phosphatase.

Prostatic acid phosphatase (PAP) is expressed in nociceptive neurons and functions as an ectonucleotidase. Injection of the secretory isoform of PAP has potent antinociceptive effects in mouse models of chronic pain. These data suggested that a small molecule activator of PAP may have utility as a novel therapeutic for chronic pain, while inhibitors could be used to acutely inhibit PAP in vitro and in vivo. To identify small molecule modulators of PAP activity, we validated a high throughput, fluorescence-based biochemical assay and then used this assay to screen a compound library. We decreased the frequency of false positive activators by subtracting compound fluorescence from the final assay fluorescence. This approach significantly reduced the number of false positive activators found in the screen. While no activators were confirmed, seven novel inhibitors of PAP were identified. Our results suggest this high throughput assay could be used to identify small molecule modulators of PAP activity.

Decorin core protein (decoron) shape complements collagen fibril surface structure and mediates its binding.

Decorin is the archetypal small leucine rich repeat proteoglycan of the vertebrate extracellular matrix (ECM). With its glycosaminoglycuronan chain, it is responsible for stabilizing inter-fibrillar organization. Type I collagen is the predominant member of the fibrillar collagen family, fulfilling both organizational and structural roles in animal ECMs. In this study, interactions between decoron (the decorin core protein) and binding sites in the d and e(1) bands of the type I collagen fibril were investigated through molecular modeling of their respective X-ray diffraction structures. Previously, it was proposed that a model-based, highly curved concave decoron interacts with a single collagen molecule, which would form extensive van der Waals contacts and give rise to strong non-specific binding. However, the large well-ordered aggregate that is the collagen fibril places significant restraints on modes of ligand binding and necessitates multi-collagen molecular contacts. We present here a relatively high-resolution model of the decoron-fibril collagen complex. We find that the respective crystal structures complement each other well, although it is the monomeric form of decoron that shows the most appropriate shape complementarity with the fibril surface and favorable calculated energies of interaction. One molecule of decoron interacts with four to six collagen molecules, and the binding specificity relies on a large number of hydrogen bonds and electrostatic interactions, primarily with the collagen motifs KXGDRGE and AKGDRGE (d and e(1) bands). This work helps us to understand collagen-decorin interactions and the molecular architecture of the fibrillar ECM in health and disease.